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Former ‘tenth planet’ may be smaller than Pluto

By Kelly Beatty, SkyandTelescope.com

The bragging rights in the outer solar system may have shifted over the weekend. If the early results hold up, this time it’s the dwarf planet Eris’s turn to be demoted, and Pluto might have just regained its status as the largest object in the Kuiper Belt, the ring of icy bodies beyond Neptune.

The scene for this drama is high in the Chilean Andes, where on Saturday three teams of astronomers caught Eris in the act of passing directly in front of a star. Astronomers had known about this occultation well in advance, but the predicted path had shifted up and down along South America, leaving astronomers unsure who – if anyone – might see the event.

Well, the news from Chile is that three observing teams, all using relatively modest telescopes, saw the star blink out. Emmanuël Jehin of the University of Liège, Belgium, tracked the event with the 60-cm TRAPPIST telescope at La Silla Observatory. “I got seven frames where the light of the star is gone,” he says. Since the images were taken 4.5 seconds apart, Jehin says the occultation lasted about 27 seconds.

Then word came that the occultation had been witnessed by two more telescopes some 740 km to the north. Sebastian Saravia, Alain Maury, and Caisey Harlingten saw the star disappear for 76 seconds through Harlingten’s 50-cm PlaneWave telescope at the San Pedro de Atacama Celestial Explorations Observatory. The outage was also recorded by a remotely operated 40-cm telescope in another dome at the SPACE site, under the control of Jose-Luis Ortiz of the Institute of Astrophysics of Andalusia, Spain.

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Tricky measurements

Any positive result would be cause enough to celebrate – never before has an occultation involved an object so far away. But successful observations from widely separated sites create two chords across Eris’s shadow that yields a unique solution for its diameter (assuming that the object is spherical).

That number, according to Bruno Sicardy of the Paris Observatory, is hard to pin down exactly because timings derived from the three telescopes’ light curves have some uncertainty. Even so, Sicardy notes in an email, “Almost certainly Eris has a radius smaller than 1170 km.” That would make it ever-so-slightly smaller than Pluto, whose radius is thought to be 1172 (+ or – 10) km. Don’t be surprised if the final value gets pushed another 50 or 60 km lower.

“This is huge,” notes occultation guru David Dunham. And you’ll get no disagreement from Mike Brown of Caltech, who headed the team that discovered Eris several years ago.

“Most of the ways we have of measuring the sizes of objects in the outer solar system are fraught with difficulties,” Brown notes on his website. “But precisely timed occultations like these have the potential to provide incredibly precise answers.”

Growing and shrinking

We’ve gotten comfortable thinking that Eris is larger than Pluto, and why not? Images taken in December 2005 by Brown and others with the Hubble Space Telescope indicated a diameter of 2400 km, just 5 per cent larger than Pluto’s. But the true size remained uncertain because even Hubble’s super-sharp gaze is only barely able to resolve Eris’s disc. (Remember&colon; it’s some 14 billion kilometres from the sun, twice as far away as Pluto.)

Observations made with the Spitzer Space Telescope yielded a diameter nearer 2600 km, and another group, using the IRAM radio telescope in Spain, upped the value to something closer to 3000 km. However, astronomers now realise that Eris’s spin axis is pointing toward the sun, an aspect that would keep the sunlit hemisphere warmer than average and skew any infrared measurements toward higher values.

The relatively brief occultation (it had been predicted to last nearly 2 minutes), combined with negative observations from a pair of telescopes in Argentina, suggested right away that Eris wasn’t as large as thought. So the outcome from Chile is actually more in line with the Hubble result from 2005.

Dense rock

Eris’s mass, determined from the orbit of its moon Dysnomia, is about 125 per cent that of Pluto – and that’s unchanged. So if the occultation results hold up, then the density of Eris must be higher, 2.5 g/cm3 or more, and its albedo (reflectivity) at least 90 per cent.

“A year ago I would have declared that result to be thoroughly crazy,” Brown says, “as it just seems unreasonable that Eris would have a density that high.” But he adds, “The albedo is already so ridiculously high that just a little more ridiculousness is OK.” Earlier this year Brown and others found that the largish Kuiper Belt denizen 50000 Quaoar is essentially a dense rock, averaging at least 3 g/cm3. “So I would say that densities, too, are up in the air,” Brown says.

One more thing&colon; occultations are very powerful probes of whether an object has an atmosphere, and for now all the team members I contacted are mum on whether Eris has one. The object’s high reflectivity, combined with its great distance, suggests that the surface must be absorbing too little sunlight to vaporise any ice present there. But during an occultation in 1988 astronomers were shocked to find that Pluto has a thin atmosphere – so stay tuned!